Free-form fabrication, abbreviated FFF, is a technique for manufacturing solid products which can have a complicated geometry, e.g. double-curved surfaces, and a complicated shape, e.g. cavities, without any other shaping than the utilisation of a CAD-system based on solids. In contrast to e.g. cutting manufacturing methods, the product is formed by means of depositing material in layers in the FFF-method.
There are a number of different FFF-methods, but only those which have certain parts in common with the present invention are discussed herein, e.g. U.S. Pat. No. 5,555,481 according to which a body is formed in layers from two different powder materials in accordance with a pattern controlled by a computer program. Each layer is compacted by means of a press device compressing the deposited powder layer. One of the powder materials forms the product itself whereas the other powder material constitutes a support for the prospective product during its fabrication. After the deposition in layers, those powder layers which are to form the product are bonded, e.g. by means of a heat treatment, while the powder in the support layers remains unaffected. The positioning of the two powder layers in geometrical patterns is very time-consuming, which makes the method commercially useless for mass-production.
WO 93/25336 discloses a process for deposition in layers of bodies from a powder, where selected portions of each layer which is to form the product are treated with thin jets of a binder which loosely binds the body together. The non-bonded powder material constitutes supporting material during the deposition in layers and is removed later. The loosely bonded body also can be subjected to a further processing in order to improve its durability.
Heat treatment or chemical reaction in layers, e.g. SLS or SLA, may be problematic. When performing e.g. laser sintering, the uppermost layer is heated intensely and simultaneously adheres to the preceding layer. When cooling down immediately afterwards, the material shrinks and shear stresses are created. These manifest themselves by means of the external corners of the component bending upwards.